Low NOx burner

ABSTRACT

A burner for use in boilers and other fired heating systems comprising an outer containment tube and an inner mix tube in a concentric arrangement. The inner mix tube serves as a mix chamber for the fuel and the oxidant. The annular space between the inner mix tube and the outer containment tube forms the fuel chamber. A plurality of holes formed through the wall of the inner mix tube provides fluid communication between the fuel chamber and the mix chamber. Oxidant flows separately into the mix chamber where the incoming fuel mixes with the oxidant. The fuel-oxidant mixture then ignites at the burner tip, combusting cleanly and efficiently and forming combustion emissions that are low in NO x .

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to combustion equipment, and more specifically to burners that provide low NO_(x) emissions.

[0003] 2. Description of the Related Art

[0004] Many industrial applications require large scale heat generation from burners in process heaters, boilers, or other fired heating systems. When the burner fuel is thoroughly mixed with combustion air under ideal conditions, the resulting combustion products are primarily carbon dioxide and water vapor. However, when the fuel is burned under less than ideal conditions, such as in the high temperature environment of boilers or process heaters, then the nitrogen that is present in the combustion air reacts with the oxygen to produce nitrogen oxides (NO_(x)). Generally, NO_(x) production increases as the temperature of the combustion process increases. It is desirable to minimize NO_(x) emissions from the combustion process because NO_(x) is generally considered to contribute to ozone depletion and other environmental problems.

[0005] Prior to the current increasing concerns over the environmental effects of NO_(x) emissions, single stage burners were commonly used for most types of fired heater applications. These single stage burners typically comprised one or more fuel nozzles or distributors positioned inside the burner wall. As compared to the typical low NO_(x), burners now in use, these single stage burners were less expensive, less complex, safer, more stable, simpler to operate, control, and maintain, and had higher turndown ratios. Unfortunately, however, these simpler single stage burners produced very high levels of NO_(x) emissions and could not operate within the demanding environmental standards and regulations now in effect.

[0006] Burners designed for combusting fuel with air in a manner resulting in less NO_(x) emissions are commonly referred to as “low NO_(x)” burners. One type of apparatus now used for reducing NO_(x) emissions is a “staged air” burner. Staged air burners operate by dividing the flow of combustion air to create a first combustion zone (wherein the fuel is introduced) having a deficiency of air so as to create a reducing environment that suppresses NO_(x) formation and a second combustion zone wherein the remaining portion of air is introduced and the combustion process is completed.

[0007] Another type of low NO_(x) apparatus is a “staged fuel” burner wherein all of the combustion air, but only a portion of the fuel to be burned, are introduced in a first combustion zone. The remaining fuel is introduced into a second combustion zone utilizing the oxygen-rich effluent of the first zone. In such a burner, the excess air in the first zone serves to dilute the fuel, which lowers the temperature of the burning gases and thereby reduces the formation of NO_(x).

[0008] Other low NO_(x) methods and apparatuses recirculate and mix furnace flue gases with fuel/air mixtures to dilute the mixtures and to thereby lower the combustion temperature so that NO_(x) formation is reduced. Flue gases are captured from the furnace space and conducted via pipes, ducts, or passageways to a mixer assembly, typically within the burner housing, where the flue gases are mixed with fuel or with fuel and air. The resulting mixture is then burned.

[0009] It will be appreciated that to accomplish the foregoing, each of the conventional types of low NO_(x) burners must be rather complex in structure and operation. As compared to high NO_(x) prior art single stage burners, conventional low NO_(x) systems must provide for and include additional hardware, conduits, passageways and other structures to achieve staged introduction of fuel or air, to allow for the burning of fuel/air mixtures in multiple combustion zones, and/or to accommodate the recirculation of furnace gases. This increased level of complexity does not lend itself to low-cost manufacture, reliability, or ease of maintenance. Moreover, in staged burner systems, the necessity of splitting flows to and balancing the performance of multiple combustion zones/stages increases the difficulty of achieving and maintaining operational stability and greatly reduces the available operating range (turndown ratio) of the burner.

[0010] One type of low NO_(x) staged fuel burner heretofore known in the art is described in U.S. Pat. No. 5,195,884. In the '884 burner, a primary portion (preferably about 75%) of the fuel gas used in the burner must be burned in a first (“primary”) combustion zone within and surrounded by the burner wall. The primary fuel is mixed with air and discharged into the primary combustion zone using one or more mixing and discharge assemblies which project into and are at least partially contained within the throat of the burner. Each of these assemblies comprises a Venturi aspirating tube having a primary fuel gas nozzle positioned at the lower end thereof.

[0011] The remaining (secondary) fuel gas used in the '884 burner is delivered to a secondary combustion zone by four secondary fuel gas nozzles outside of the burner wall. Each of the secondary fuel nozzles has an array of multiple flow ports provided therein which must spread the secondary flue gas in a fan-type pattern covering essentially one-quarter of the exterior of the burner wall. The burner wall has an exterior frusto-conical surface which is contacted by the secondary fuel gas as it spreads outwardly and moves upwardly to the secondary combustion zone. Such contact is said to promote the mixing of internal flue gases with the secondary fuel gas.

[0012] Unfortunately, the primary combustion stage alone of the burner described in the '884 patent produces more NO_(x) emissions than are allowable, for example, under Texas Gulf Coast restrictions and other regulatory requirements. Moreover, without using the primary combustion stage, it has not been possible heretofore to obtain adequate heating or to achieve and maintain stable burner operation.

[0013] Like the stage fuel burner described in the '884 patent, few, if any, of the other low NO_(x) burners presently available are capable of meeting Texas Gulf Coast requirements and other increasingly stringent air quality standards. If suitable new burner technologies capable of satisfying these requirements are not found, the industry will be required to use more expensive and elaborate techniques, such as catalytic reduction, to reduce NO_(x) emissions.

[0014] Thus, a need exists for a new burner technology that produces even less NO_(x) emissions than the low NO_(x) burner systems currently available in the art. The new, extremely low NO_(x) burner would preferably also be less complex, less expensive, more stable, and much simpler to operate, maintain, and control than current low NO_(x) burner systems. Further, the new, extremely low NO_(x) burner would preferably provide a much larger available turndown ratio than is provided by current low NO_(x) burners.

SUMMARY OF THE INVENTION

[0015] The present invention provides a burner for use in gas burning heaters and boilers. The burner includes an outer containment tube and an inner mix tube that together form an annular space, wherein the annular space is sealed at a first end and at a second end. The burner further includes a mix chamber formed within the inner mix tube and a plurality of holes in fluid communication between the annular space and the mix chamber. Fuel passes through the plurality of holes to mix with an oxidant in the mix chamber forming a fuel-oxidant mixture. The outer containment tube and the inner mix tube are parts of a flame assembly.

[0016] The plurality of holes may be formed tangentially through the wall of the inner mix tube to impart a spin on the fuel as the fuel enters the mix chamber. Each of the plurality of holes may be formed at the same angle through the inner mix tube or the angle may vary between different holes in the plurality of holes. The angles the holes are formed may be between about 0 degrees and about 70 degrees from the radius, or between about 10 degrees and about 50 degrees from the radius, or between about 15 degrees and about 30 degrees from the radius.

[0017] While the plurality of holes may be randomly arranged, the holes may also be arranged in spirals, rows, columns, circles, triangles and combinations thereof. The plurality of holes may be arranged in staggered columns arranged symmetrically around the inner mix tube.

[0018] The oxidant flows into the mix chamber, normally either by natural or induced draft of by forced draft. The gas flows into the mix chamber through the plurality of holes drilled at an angle inducing a swirling vortex flow in the mix chamber.

[0019] The oxidant is normally air but it may be any substance having sufficient oxygen to support combustion of the fuel gas. The fuel is a gas, typically comprising natural gas, hydrogen, methane, ethane, propane, butane, propylene, butylene and combinations thereof.

[0020] The outer containment tube and the inner mix tube may be made of many materials known to those having ordinary skill in the art. Suitable materials include, for example, stainless steel, titanium and combinations thereof.

[0021] The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of a preferred embodiment of the invention, as illustrated in the accompanying drawing wherein like reference numbers represent like parts of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022]FIG. 1 is an exploded view of the burner in accordance with the present invention.

[0023]FIG. 2 is a top view of the burner operating in accordance with the present invention.

[0024]FIG. 3 is a cross sectional drawing of the flame assembly in accordance with the present invention.

[0025] FIGS. 4A-B are a pattern showing a layout of the plurality of holes drilled in the walls of the mix chamber in accordance with the present invention.

DETAILED DESCRIPTION

[0026] The present invention provides a burner for use in boilers, hot water heaters and other fired heating systems. Advantageously, the burner produces combustion emissions that are low in nitrogen oxides (NO_(x)). The burner has one or more flame assemblies comprising an outer containment tube and an inner mix tube. The inner mix tube serves as a mix chamber for the fuel and the oxidant. The annular space that is formed between the inner mix tube and the outer containment tube is the fuel chamber. A plurality of holes formed through the wall of the inner mix tube provides fluid communication between the fuel chamber and the mix chamber. Oxidant flows separately into the mix chamber where the incoming fuel mixes with the oxidant. The fuel-oxidant mixture then ignites at the burner tip, combusting cleanly and efficiently and forming combustion emissions that are low in NO_(x).

[0027] The burner may be oriented upwardly, downwardly, horizontally or generally at any other desired operating angle. The fuel is gaseous and may be natural gas or any other type of fuel gas blend employed in process heaters, boilers, or other gas fired heating systems. Typical components of a fuel gas blend may include, for example, hydrogen, methane, ethane, propane, propylene, butane, butylene and combinations thereof.

[0028] The fuel chamber, formed in the annular space between the outer containment tube and the inner mix tube, further comprises a fuel port through which fuel is supplied to the fuel chamber. Preferably, if the burner has more than one flame assembly, the fuel ports of the flame assemblies may be connected to a manifold and the manifold connected to the fuel source.

[0029] Oxidant is provided to the mix chamber by a controlling mechanism. The oxidant may be, for example, air, oxygen enriched air, or other gases containing sufficient amounts of oxygen as the oxidant. The oxidant may be supplied under pressure, as with a blower or a compressor, or the oxidant may be induced into the burner by the draft of the boiler. The amount of oxidant entering the mix chamber is controlled by a controlling mechanism, which may be, for example, dampers, shutters or an adjustable plate that partially blocks the oxidant flow when less oxidant is needed. The adjustment may be made manually or the adjustment may be made by a controller monitoring, for example, stack O₂ level, flame stability, flame temperature and other relevant control points known to those having ordinary skill in the art.

[0030] The oxidant flowing through the mix chambers of the flame assemblies does not necessarily include all the oxidant required for combustion. Other oxidant may flow around the burner assembly and provide needed oxidant at the burner tip to complete the combustion process. The oxidant flowing through the mix chamber may range between about 20% and about 120% of that required for complete combustion. Preferably, the oxidant flowing through the mix chamber ranges from about 50% to about 105% of that required for complete combustion. Most preferably, the range for the oxidant flowing through the mix chamber will range from about 75% to about 103% of the amount of oxidant required for complete combustion. Oxidant supplied to the burner tip, bypassing the mix chamber, may be supplied by methods well known to those having ordinary skill in the art such as by natural or induced draft of the furnace or by forced draft from a fan, blower or compressor.

[0031] Each of the plurality of holes formed through the wall of the mix chamber is formed at an angle so that the fuel is introduced into the mix chamber in a swirling or vortex action. The swirling action mixes the fuel with the oxidant flowing through the mix chamber. Each of the holes are generally formed by drilling through the wall of the mix chamber, but other means of forming the holes, such as casting or molding the holes when the inner mix tube is formed, are also acceptable. The angle of each of the holes is measured as the circumferential angle in the plane cutting the inner mix tube at its circumference. The angle, measured from the mix tube's radius, may range between about 0 degrees and about 70 degrees. Preferably, the angle may range between about 10 degrees and about 50 degrees. Most preferably, the angle may range between about 15 degrees and about 30 degrees. An angle of 0 degrees from the centerline of one of the plurality of holes would place the centerline at the radius.

[0032] The plurality of holes may be arranged randomly or with order. The plurality of holes may be arranged in spirals, rows, columns, circles, or combinations thereof. One embodiment provides the plurality of holes arranged in two or more staggered columns down the sides of the inner mix tube.

[0033] The number of holes required is determined by the minimum and maximum allowable pressure drop. If not enough holes are formed, or if the diameter of each hole is too small, then the fuel will not be able to flow into the mix chamber and the burner duty will be limited. If the are too many holes or if the diameter is too large, then the fuel will not swirl adequately to ensure low nitrogen oxide emissions. The diameter of the mix tube is preferably between about one half inch and four inches. Preferably, the diameter of the mix tube is between about one inch and three inches. The diameter of the mix tube must be large enough to allow the oxidant and fuel to flow through the mix tube with a pressure drop low enough so as not to limit the oxidant required for combustion.

[0034] The swirling motion of the oxidant-fuel mixture creates a swirling flame pattern upon ignition. The swirling flame pattern is thought, without limiting the invention, to create a mixing of the flue gas with the mixture, thereby reducing the amount of NO_(x) contained in the combustion emissions. When a burner of the present invention is formed with more than one flame assembly, the direction of the swirling pattern is reversed for each of the assemblies to further induce mixing of the flue gas into the flames swirling in opposite directions, resulting in a lower NO_(x) emissions level.

[0035] The burner may be made of any material that is capable of withstanding the temperatures the burner is exposed to from the combustion of the fuel. Stainless steel is a preferred material of construction.

[0036]FIG. 1 is an exploded view of a burner in accordance with the present invention. The fully assembled burner 10 includes four flame assemblies 11. The number of flame assemblies making up a burner is dependant on the required fired duty for the burner. The flame assemblies 11 include the outer containment tube 12 and the inner mix tube 13. The inner mix tube 13 includes a plurality of holes 14 formed through the wall of the inner mix tube 13. The fuel chamber 15 is formed in the annular space between the outer containment tube 12 and the inner mix tube 13. The burner 10 is connected to the fuel source at the burner fuel inlet 15. The burner fuel inlet 15 may be connected to the fuel source with a flange or with a threaded connection.

[0037] Fuel may flow to the fuel chambers 15 through a fuel manifold 17. A fuel port 16 is in fluid communication with the fuel manifold 17 and the fuel chamber 15. Fuel may flow from the fuel manifold 17 to the fuel chamber 15 through the fuel port 16. Fuel may then flow from the fuel chamber 15 to the mix chamber 18 through the plurality of holes 14. The plurality of holes 14 are formed at a circumferential angle to impart a spin on the fuel as the fuel enters the mix chamber 18 from the fuel chamber 15.

[0038] The outer containment tube 12 and the inner mix tube 13 may be supported and sealed by a flame assembly base 22 in the shape of a ring. Because the pressure is normally very low in the burner, gaskets or other sealing materials are not generally necessary. The inner mix tube 13 is supported and sealed by a flange 20 at the lower end of the flame assembly ring base 22. The outer containment tube 12 is supported and sealed in a groove 19 formed in the top end of the flame assembly base 22. Support rods 21 are threaded or otherwise sealed in holes 24 and then fastened on the other end through the frame member 23. The frame member 23 provides a seal on the flame end of the flame assembly 11 and is held in place by the support rods 21.

[0039] Oxidant flows through the oxidant inlet 23 in the center of the flame assembly base 22. Oxidant flows through the oxidant inlet 23 into the mix chamber 18. An adjustable plate or cone 25 controls the oxidant flow to the mix chamber 18. The adjustable cone 25 slides along the vertical supports 27 on two slots 20 on opposite ends of the adjustable cone 25. A crossbar 30 supports a threaded rod 29 passing through the adjustable cone 25. A nut 26 supports the adjustable cone 25 at the burner end of the threaded rod 29 and a spring 28 biases the adjustable cone 25 towards the oxidant inlet 23. As the adjustment nut 31 is turned, the adjustable cone 25 slides along the vertical supports 27, thereby controlling the oxidant flow into the mix chamber 18.

[0040]FIG. 2 is a top view of the burner operating in accordance with the present invention. The burner 10 is enclosed by the burner throat 38 of the boiler 32. Preferably the burner 10 is positioned in the burner throat 38 so that the flame end of the burner 10 is just even with the end of the burner throat 38. Combustion air 37 flow rate may be controlled by shutters 35 that are manually controlled or controlled automatically by an actuator 36 as known to those having ordinary skill in the art. Each flame assembly 11 has an individual damper 34 to adjust air flow to the mix chamber. Optionally, the individual dampers may be individually controlled by a controller and actuator as known by those having ordinary skill in the art.

[0041]FIG. 3 is a cross sectional drawing of the flame assembly 11 in accordance with the present invention. The outer containment tube 12 and the inner mix tube 13 form the annular fuel chamber 15. Oxidant flows through the mix chamber 18 and fuel flows from the fuel chamber 15 to the mix chamber 18 through the plurality of holes 14. Because the plurality of holes 14 are drilled at an angle, the fuel imparts a swirling action or a vortex in the mix chamber 18.

EXAMPLE 1

[0042] A burner was constructed and then tested in a boiler. The burner included four flame assemblies. Each flame assembly had an outer containment tube that was 2.5 inches in diameter and 3.25 inches tall. Each inner mix tube was 2.25 inches in diameter with eight columns of staggered holes drilled through the wall of the mix chamber. Each column had 4 holes having a 0.156 inch diameter drilled at an angle of 20 degrees from the radius. Two of the inner mix tubes were drilled at an angle of 20 degrees to induce a clockwise swirl and two of the inner tubes were drilled at an angle of 20 degrees to induce a counterclockwise swirl. FIGS. 4A-B are a pattern that may be used for a layout of the plurality of holes drilled in the walls of the mix chamber in accordance with the present invention. Each of the plurality of holes 14 was formed in the wall of the inner mix tube 13 in columns, each column having four holes 14. The holes 14 were arranged in a staggered pattern as shown in FIG. 4A. FIG. 4B shows the angle 52 measured against the radius 51 for forming each of the holes drilled to form a clockwise swirl. The angle would be measured on the opposite side of the radius 51 to form a counterclockwise swirl.

[0043] The burner was installed in a commercial boiler used to generated steam for heating a large building. The fuel was natural gas supplied from the local public gas utility. The fuel gas pressure at the burner was 3.5 psi with a gas rate of about 12,500 SCFH. The firebox temperature was about 2312° F. and the stack temperature was about 354° F. The flue gas analysis was as follows: O₂—4.2%; CO₂—4.2%; SO_(x)—0%; NO_(x)—16 ppm; and CO—0%.

[0044] It will be understood from the foregoing description that various modifications and changes may be made in the preferred embodiment of the present invention without departing from its true spirit. It is intended that this description is for purposes of illustration only and should not be construed in a limiting sense. The scope of this invention should be limited only by the language of the following claims. 

What is claimed is:
 1. A burner, comprising: an outer containment tube; an inner mix tube forming an annular space with the outer containment tube, wherein the annular space is sealed at a first end and at a second end; a mix chamber formed within the inner mix tube; and a plurality of holes in fluid communication between the annular space and the mix chamber, wherein fuel passes through the plurality of holes to mix with an oxidant in the mix chamber forming a fuel-oxidant mixture, wherein the outer containment tube and the inner mix tube form a flame assembly.
 2. The burner of claim 1, wherein the plurality of holes are formed tangentially.
 3. The burner of claim 1, wherein each of the plurality of holes is formed at the same angle through the inner mix tube.
 4. The burner of claim 3, wherein the angle is between about 0 degrees and about 70 degrees from the radius.
 5. The burner of claim 3, wherein the angle is between about 10 degrees and about 50 degrees from the radius.
 6. The burner of claim 3, wherein the angle is between about 15 degrees and about 30 degrees from the radius.
 7. The burner of claim 1, wherein the fuel flowing through the plurality of holes mixes with the oxidant within the mixing chamber.
 8. The burner of claim 1, wherein the fuel flowing through the plurality of holes induces a vortex with the oxidant and fuel within the mixing chamber.
 9. The burner of claim 1, wherein the fuel flowing through the plurality of holes creates intimate mixing of the oxidant and fuel in the mix chamber.
 10. The burner of claim 1, further comprising a fuel port in fluid communication with the annular space.
 11. The burner of claim 1, further comprising an oxidant port in fluid communication with the mixing chamber.
 12. The burner of claim 8, wherein the oxidant is air.
 13. The burner of claim 1, wherein the fuel is a gas.
 14. The burner of claim 13, wherein the fuel is selected from natural gas, hydrogen, methane, ethane, propane, butane, propylene, butylene and combinations thereof.
 15. The burner of claim 1, wherein the fuel oxidant mixture ignites at the second end of the burner.
 16. The burner of claim 1, wherein the plurality of holes are arranged in one or more columns.
 17. The burner of claim 1, wherein the plurality of holes are randomly arranged.
 18. The burner of claim 1, wherein the plurality of holes are arranged in an order.
 19. The burner of claim 9, wherein the order is selected from spirals, rows, columns, circles, triangular and combinations thereof.
 20. The burner of claim 1, wherein the outer containment tube and the inner mix tube are formed of materials selected from carbon steel, stainless steel, titanium and combinations thereof.
 21. The burner of claim 1, further comprising an air port in fluid communication with the mix chamber.
 22. The burner of claim 1, wherein the air flows into the air port by a draft.
 23. The burner of claim 21, wherein the air flows into the air port by forced means, wherein the forced means is selected from a blower and a compressor.
 24. The burner of claim 1, further comprising: a base adapted for sealing the first end of the annular space; and an annular cap for sealing the second end of the annular space.
 25. The burner of claim 1, wherein the burner comprises more than one flame assemblies. 